The present invention relates to a thermal print head and more particularly to a fault detection circuit adapted for detecting and isolating failures or faults in any of the heating elements or heating circuits contained in the thermal print head.
Conventional print heads of thermal printers contain a plurality of heating circuits, each comprised of a heating element and a gating circuit for controlling when electrical current passes through the heating element. Each gating circuit typically contains transistors, logic gates and the like. A failure in any one component of any given heating circuit will disable part of the dot matrix pattern that is output during printing, rendering the print head nonfunctional. The most frequent cause for failures results from circuit line breakage in the heating elements.
It has been proposed, as described for example in Japanese Laid-Open Patent Application No. 58 (1983) - 28391, to sequentially direct a low test current that is not high enough for printing, through each of the heating circuits. According to the method, a fault is detected if the current does not pass through any one of the heating circuits. The method is implemented with the aid of resistors having a low resistance value which are connected in series between the heating circuits and a common terminal side of the print head. When current is sequentially supplied to each of the heating circuits, a voltage across the resistors is sensed to determine whether current has passed through the heating circuits.
The above approach suffers from the drawback that in order to detect the very low test currents, an amplification circuit of a high amplification factor is needed to permit sensing of the test currents. Frequent adjusting of the amplification factor is also necessary. As a result, the testing circuitry becomes complicated and the time for completing a test is sufficiently long to constitute a real drawback of this checking process. Furthermore, because the testing relies on the value of the resistances and because manufacturing variations produce inconsistent resistance values in the heating elements themselves, it is necessary to carry out fine adjustment of the amplification factor for each thermal print head. It is difficult to simultaneously accommodate all of the variations in all of the resistances of all of the heating elements in any given thermal print head.